Hi Jeanny,
Thank you for buying our theme. We hope it will meet all your expectations! 🙂
Be aware that the register form is managed by BuddyPress plugin. In that case, we think this issue is related with BuddyPress and it might be best if you try to reach the plugin authors as they will be more informed and better suited to help you: buddypress.org/support.
However, we took a look and tried to register on mobile but it seems the registration process was working correctly. 🤔
We remain available for all support requests related with the use of your theme.
Best,